For PET fibers commercially available, the highest strength so far is about 1.1 GPa and the empirical highest strength is no more than 3 to 4% of the theoretical highest strength, which is one third of the strength of other high strength fibers (e.g., ultimate-performance para-aramid (Kevlar) fiber having the strength of about 2.9 GPa). The use of the PET fibers as a fiber material is thus limited in the fields of industrial applications that require ultimate performance, other than general clothing or household or limited industrial (tire cords) applications.
Non-LC thermoplastic fibers, such as PET and nylon, display lower strength than LCP (Liquid Crystal Polymer) fibers, such as PBO (Zylon) or para-aramid (Kevlar) fibers, and their empirical strengths are impossible to increase dramatically with respect to the theoretical strengths. The reason lies in the difference of the structure-forming behavior while the resin is being processed into fiber.
Due to its liquid crystalline structure in the solution state, the LCP fiber has a small entropy difference in the fiber structure before and after the spinning process under appropriate shear stress and forms a fiber structure having a considerably high degree of orientation and crystallinity, so it can be made into high strength, high-performance fibers.
In contrast, the non-LC thermoplastic polymers like PET or nylon in molten state have a complicated structure with the polymer chains entangled in the form of amorphous random coils, so they are relatively hard to form with complete orientation and crystallization (i.e., high strength) due to their entangled structure in the form of random coils even if they are under a high shear stress in the spinning nozzle and stretched at an elongation ratio (draft and elongation ratio, etc.) out of the spinning nozzle. For this reason, there is a large entropy difference of the fiber structure before and after the spinning process.
Despite the structural demerits of general-purpose thermoplastic polymers, the PET fiber having a relatively high strength with respect to the existing fibers is expected to extend the market of its applications and to start an enormous ripple effect through the industry. In recent years, a variety of studies have been made in the Japanese textile industries to maximize the properties of the existing general-purpose PET fiber and to increase the critical performance of the fiber.
The subjects of the recent researches concerning the high strength PET fibers include, for example, the use of ultra-high molecular PET resins [Ziabicki, A., “Effect of Molecular weight on Melt Spinning and Mechanical Properties of High-Performance Poly(ethylene terephthalate) Fibers”, Test. Res. J., 1996, 66, 705-712; Sugimoto, M., et al., “Melt Rheology of Polypropylene Containing Small Amounts of High-Molecular-Weight Chain. 2. Uniaxial and Biaxial Extensional Flow”, Macromol., 2001, 34, 6045-6063] and the use of the coagulation bath technique in the melt spinning process to maximize the orientation [Ito M., et al., “Effect of Sample Geometry and Draw Conditions on the Mechanical Properties of Drawn Poly(ethylene terephthalate)”, Polymer, 1990, 31, 58-63].
The above studies are to develop high strength PET fibers in a small-scaled laboratory, so no commercialization is allowed owing to the limitation in the workability and productivity with respect to the effect of the improvement of physical properties.
It has recently been reported that Japanese scientists are on the progress of research and development using general-purpose thermoplastic polymers like PET, nylon, etc. to increase the strength of the existing fibers from 1.1 GPa to 2 GPa within a range that does not raise the production cost more than twice in terms of the melt spinning process.
Furthermore, the ongoing research and development technologies in progress for the purpose of applying them for practical uses in the tire cords most consumed as an industrial fiber as soon as possible focus on the following technologies: molten structure control, molecular weight control, draw/heating, and evaluation/analysis.
Unlike the conventional technologies that realize fibers with high strength by the control of the fiber structure formation behavior through molecular orientation and crystallization of solidified fibers, the molten structure control technology in particular involves an approach to the control of the molecular entanglement structure in a molten polymer and focuses on the PET fibers having a high strength by studying the control of the structure and behavior in the non-oriented amorphous fibers.
There has been reported the development of high strength PET fiber through the design of spinning nozzles, laser heating, supercritical gas, coagulation bath, etc. as a means to control the molecular structure in the melt spinning process.
In particular, a conventional method of designing spinning nozzles used in the melt spinning process is adopted to produce high strength PET fibers through a localized heat-up process in the vicinity of the spinning nozzle. For examples, FIG. 7 shows an embodiment of a localized heating process performed right under the spinning nozzle, and FIG. 8 is a cross-sectional view of the embodiment of the localized heating process taken along the line III-III of FIG. 7.
More specifically, in the melt spinning process, a spinning nozzle 100 is fixed to a pack body 200 held by a pack-body heater 300 with a heat source of 100 to 350° C. After the spinning process, the multifilament passes through an annealing heater 400 having a thickness of 20 to 200 mm to maintain a constant distance from an electric heater having a temperature ranging from the room temperature to high temperature of 400° C., thereby achieving thermal transfer with high efficiency at a lower cost.
The localized heating on the fiber with the annealing heater 400 is not for heating the fiber but for warming the fiber to maintain the uniform temperature of the holes in the bottom of the spinning nozzle. Due to the minimization of the temperature variations of the holes, it is possible to improve the spinning workability and the product quality at once. But the distance between the fiber and the heater is too long, and a uniform heating is not applied to the fiber.
Another conventional method of performing a localized heating in the vicinity of the nozzle during the melt spinning process involves the irradiation of CO2 laser beams right under the spinning nozzle with holes having a micro-sized diameter to prepare a high-performance PET fiber having strength of 1.68 Gpa (13.7 g/den) and elongation of 9.1% after drawing [Masuda, M., “Effect of the Control of Polymer Flow in the Vicinity of Spinning Nozzle on Mechanical Properties of Poly(ethylene terephthalate) Fibers”, Intern. Polymer Processing, 2010, 25, 159-169].
In this regard, FIG. 9 is an embodiment of the localized heating by laser beams right under the spinning nozzle, and FIG. 10 is a cross-sectional view of the embodiment taken along the line IV-IV of FIG. 9.
More specifically, multifilament 112 are directly heated with CO2 laser beams from a laser source 410 after the spinning process, with the bottom of a spinning nozzle 100 projecting to the bottom end of a pack body 200 to a length of 1 to 3 mm, and the CO2 laser beams are irradiated from a distance of 1 to 10 mm immediately after the spinning process.
The laser heating process right under the spinning nozzle makes a specific portion of the fiber heated up to high temperature, but it is difficult to use for a commonly used spinning nozzle having dozens to tens of thousands of holes.
In an attempt to solve the problems with the conventional preparation method for high strength synthetic fiber, the inventors of the present invention have found out the fact that the optimization of the thermal transfer using a double heating method in the vicinity of capillary of a commonly-used spinning nozzle and right under the spinning nozzle can raise the temperature of the molten fiber higher than that of a pack body in a short period of time during which no degradation occurs, so as to effectively control the molecular entanglement structure in the polymer without reducing the molecular weight and to improve the mechanical properties of the synthetic fiber, such as strength, elongation, etc., thereby completing the present invention.